Even if progress is regressing it doesn't necessarily mean that the new people aren't getting anything out of the wipes, which is still beneficial? If a dude doesn't get a clear, at least he can carry over his experience to the next day he attempts it and so on.
Usually if progression is regressing, it means one of two things. Either the new people
aren't getting experience from it, or the group as a whole is getting fatigued. The former can lead to bad habits if not sorted out, and is usually better handled by a calm one-on-one talk with the person to find out the specifics of what they're doing wrong. The later is inevitable over the course of a session.
It's really something that has to be judged on a group basis. Someone who starts off getting hit by every other landslide or plume, but by the end of a run can make it to the post-heart phase without dying every time has definitely gained experience. On the other hand, the person still getting hit by landslides after three hours either has a connection issue or they're never going to get it. They've gained little to no experience, and everyone has largely wasted their time.
We had a dragoon in one our raid groups who was consistently making mistakes on Rafflesia due to nervousness. It took discussion it with the raid lead in private to eventually get him past it. A significant chunk of the "practice" he put in early on was actually completely wasted.
i think it really just boils down to how much you actually want to beat it. Some people simply might not have the time or interest to keep failing over and over again. Sometimes, just being able to fight in that battle with friends might be good enough.
The right mindset is definitely one of the biggest factors for progression of a group. If you go in thinking you'll fail, then you'll definitely fail. If you go in nervous because of preconceived notions, you'll affect your performance negatively, and possibly the performance of others. If you don't think positively of your group, it will show in your actions.
This is where static, FC, and learning groups can come in handy. You can build confidence without fear of shame, you can practice your reaction times, and you can ask for help with learning good habits/job tricks.